<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/jquery/3.4.1/jquery.js"></script>
</head>
<body>
    <div id="app">
        <div class="item">
            html
        </div>
        <div>css</div>
    </div>
    <script>
      //  find方法
      var item = $("#app").find(".item")
      console.log(item);

        // filter方法
      var i = $("#app div").filter(".item")
      console.log(i);
    </script>
</body>
</html>